Method of temperature compensation in high power focusing system for EUV LPP source

ABSTRACT

Method of temperature compensating a focusing system in which a temperature of a thermal lens compensation plate is regulated based on an optical absorption of the thermal lens compensation plate with optical absorption being determined based at least in part on an expected end-of-lifetime value for focus lens optical absorption. A value representative of cumulative time in use of the focusing systems is determined and the temperature of the thermal lens compensation plate is increased to a temperature based at least in part on said cumulative time in use.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/017,882, filed Sep. 4, 2013, now issued as U.S. Pat. No. 9,280,053 onMar. 8, 2016. The disclosure of the priority application is incorporatedin its entirety herein by reference.

FIELD

The present disclosure relates to temperature compensated focusing oflaser radiation from a laser source in a system for producing extremeultraviolet radiation using a laser produced plasma.

BACKGROUND

Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiationhaving wavelengths of around 50 nm or less (also sometimes referred toas soft x-rays), and including light at a wavelength of about 13.5 nm,can be used in photolithography processes to produce extremely smallfeatures in substrates such as silicon wafers. Here and elsewhere, itwill be understood that the term “light” is used to encompasselectromagnetic radiation regardless of whether it is within the visiblepart of the spectrum.

Methods for generating EUV light include converting a source materialfrom a liquid state into a plasma state. The source material preferablyincludes at least one element, e.g., xenon, lithium or tin, with one ormore emission lines in the EUV part of the spectrum. In one such method,often termed laser produced plasma (“LPP”), the required plasma can beproduced by using a laser beam to irradiate a source material having therequired line-emitting element.

One LPP technique involves generating a stream of source materialdroplets and irradiating at least some of the droplets with laser light.In more theoretical terms, LPP light sources generate EUV radiation bydepositing laser energy into a source material having at least one EUVemitting element, such as xenon (Xe), tin (Sn), or lithium (Li),creating a highly ionized plasma with electron temperatures of several10's of eV.

The energetic radiation generated during de-excitation and recombinationof these ions is emitted from the plasma in all directions. In onecommon arrangement, a near-normal-incidence mirror (often termed a“collector mirror” or simply a “collector”) is positioned to collect,direct (and in some arrangements, focus) the light to an intermediatelocation. The collected light may then be relayed from the intermediatelocation to a set of scanner optics and ultimately to a wafer.

In some LPP systems each droplet is sequentially illuminated by multiplelight pulses. In some cases, each droplet may be exposed to a so-called“pre-pulse” and then to a so-called “main pulse.” It is to beappreciated, however, that the use of a pre-pulse is optional, that morethan one pre-pulse may be used, that more than one main pulse may beused, and that the functions of the pre-pulse and main pulse may overlapto some extent.

Typically, a pre-pulse may cause some or all of the source material toheat, expand, gasify, vaporize, ionize, generate a weak plasma, orgenerate a strong plasma, or some combination of these, and a main pulsemay convert most or all of the material affected by the pre-pulse intoplasma and thereby produce an EUV light emission. Pre-pulsing mayincrease the efficiency of the source material/pulse interaction due toa larger cross-section of material that is exposed to the main pulse, agreater penetration of the main pulse into the material due to thematerial's decreased density, or both. Another potential benefit ofpre-pulsing is that it may expand the target to the size of the focusedmain pulse, allowing all of the main pulse to participate in conversionof the source material into a plasma. This may be especially beneficialif relatively small droplets are used as targets and the irradiatinglight cannot be focused to the size of the small droplet. Thus, in someapplications, it may be desirable to use pre-pulsing to increaseconversion efficiency and/or allow use of relatively small, e.g.,so-called, mass limited targets. The use of relatively small targets, inturn, may be used to lower debris generation and/or reduce sourcematerial consumption.

The main pulse and the pre-pulse, if it is used, must generally bedirected and focused onto the droplets of source material by using afocusing system. Generally in a high power focusing system for an EUVLPP source the optical elements in the beam path absorb some of theenergy from the main pulse and the pre-pulse causing heating in thoseoptical elements. This heating creates a thermally-induced opticaldistortion (also known as a “thermal lens”) by virtue of the temperaturedependence of the refractive index and the coefficient of thermalexpansion of the material making up the optical element. The thermallens alters the optical power of the optical elements causing deviationsfrom optimal values.

It would thus be advantageous to be able to negate the effects ofthermal lensing and so to avoid the performance limitations it imposes.

SUMMARY

The following presents a simplified summary of one or more embodimentsin order to provide a basic understanding of the embodiments. Thissummary is not an extensive overview of all contemplated embodiments,and is not intended to identify key or critical elements of allembodiments nor delineate the scope of any or all embodiments. Its solepurpose is to present some concepts of one or more embodiments in asimplified form as a prelude to the more detailed description that ispresented later.

In one aspect, there is provided an apparatus for and method oftemperature compensating a focusing system in which the focusing systemhas at least one transmissive optical element having a thermal lens. Areflective optical element is added to the system that has a thermallens that is complementary to the thermal lens of the transmissiveoptical element so that the optical characteristics of the two opticalelements combined are substantially temperature independent. Therespective thermal lenses of the two optical elements are balanced byselecting materials for the reflective optical element that have thecorrect optical absorption based on the absorption of the transmissiveoptical element and the relative strengths of the thermal lenses.Provision can also be made for a change in the absorption of thetransmissive optical element over time by selecting a value for theabsorption of the reflective optical element that exceeds acontemporaneous value for the absorption of the transmissive opticalelement and then cooling the reflective optical element to reduce thestrength of its thermal lens, with provision for increasing thetemperature of the reflective optical component over time. The focusingsystem may also include a pulse combiner for combining pulses frommultiple sources. The focusing system is especially applicable tosystems for generating EUV light for use in semiconductorphotolithography.

In another aspect, a focusing system includes a transmissive opticalelement arranged to receive and focus a light beam, the transmissiveoptical element having a first thermal lens power having a first valueand a first absorption, and a reflective thermal lens compensating platearranged to receive the light beam at least indirectly from a light beamsource and reflect the light beam before the light beam reaches thetransmissive optical element, the reflective thermal lens compensatingplate having a second thermal lens power having a second value and asecond absorption, wherein the sign of the first value and the secondvalue are opposite to one another and wherein a material for thereflective thermal lens compensating plate is chosen so that the secondabsorption relates to the first absorption and a relative absolutemagnitude of the first value and the second value such that a thermallens of the thermal lens compensating plate substantially completelycompensates for a thermal lens of the transmissive optical element. Thetransmissive optical element and the reflective thermal lenscompensating plate preferably both include the same material, which maybe zinc selenide. Other materials, for example, potassium chloride,sodium chloride, potassium bromide, diamond, etc. can also be used. Thereflective thermal lens compensating plate may have a reflectivecoating, in which case the second absorption is selected by selectingthe absorption of the coating. The reflective coating may be on a frontsurface or a back surface of the reflective thermal lens compensatingplate. The reflective coating may include gold, copper, or molybdenum.

The focusing system may further include a pulse combiner in arrangementsusing a main pulse and a pre-pulse. In systems where the pre-pulse andthe main pulse propagate along a common beam path before impinging onthe pulse combiner and the pulse combiner may comprise an optical wedge.In such systems in which the pre-pulse is generated by a CO₂ laser andthe optical wedge may comprise ZnSe with a dichroic coating, with afront surface that is highly reflective to the main pulse and a backsurface that is highly reflective to the pre-pulse. In such systems inwhich the pre-pulse is generated by a YAG laser, the optical wedge maycomprise fused silica or ZnSe. The back surface of the optical wedge maybe curved to compensate for chromatic aberrations in the final focuslens.

In systems in which the pre-pulse and the main pulse propagate alongseparate beam paths before impinging on the pulse combiner and thepre-pulse is generated by a CO₂ laser, the pulse combiner may compriseZnSe with a maximum metal reflector coating. In systems in which thepre-pulse and the main pulse propagate along separate beam paths beforeimpinging on the pulse combiner and the pre-pulse is generated by a YAGlaser the pulse combiner may comprise a material having a very lowcoefficient of thermal expansion such as fused silica or atitania-silicate glass or a lithium aluminosilicate glass-ceramic. Alsoin such systems the pulse combiner may comprise a coating having highreflectivity at about 10.6 μm and high transmissivity at about 1.064 μm.

In another aspect, the focusing system includes a transmissive opticalelement arranged to receive and focus a light beam, the firsttransmissive optical element having a first thermal lens power having afirst value and a first absorption, the first absorption increasing froma first absorption value to a second absorption value as a cumulativeamount of time the first transmissive element is used to focus the lightbeam increases, and a reflective thermal lens compensating platearranged to receive the light beam at least indirectly from a light beamsource and reflect the light beam towards the transmissive opticalelement; the first transmissive element having a second thermal lenspower having a second value and a second absorption, in which the signof the first value and the second value are opposite to one another andin which the second absorption is selected based on the secondabsorption value and a relative absolute magnitude of the first valueand the second value. The system further includes a cooling element inthermal communication with the thermal lens compensating plate, thecooling element controlling the temperature of the thermal lenscompensating plate such that a thermal lens of the thermal lenscompensating plate substantially completely compensates for a thermallens of the transmissive optical element so that the opticalcharacteristics of the focusing system are substantially independent oftemperature within the operating temperatures of the system.

The second absorption value may be an expected end-of life value for thetransmissive optical element. The cooling element may be configured as acooling plate in thermal communication with the thermal lenscompensating plate. The cooling element may include a control system forcontrolling a temperature of the thermal lens compensating plate bycontrolling a temperature of the cooling plate. The control system maycontrol the temperature of the cooling plate based on an amount of time.The cooling plate may be separated from the thermal lens compensatingplate by a gap. The control system may control the temperature of thethermal lens compensating plate by controlling a thickness of the gap.

In another aspect, a method of temperature compensating a focusingsystem is provided, the method including the steps of regulating atemperature of a thermal lens compensation plate to a first temperature,the first temperature being based on an optical absorption of thethermal lens compensation plate, the optical absorption having beendetermined based at least in part on an expected end-of-lifetime valuefor focus lens optical absorption for a final focus lens includes partof the focusing system, determining a value representative of cumulativetime in use of the focusing system, and adjusting the temperature of thethermal lens compensation plate to a second temperature higher than thefirst temperature, the second temperature having a value based at leastin part on the cumulative time in use. The adjusting step may be carriedout periodically or continuously.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic not-to-scale view of an overall broadconception for a laser-produced plasma EUV light source system accordingto an aspect of the present invention.

FIG. 2 is a diagrammatic not-to-scale view of a first embodiment of alaser source according to one aspect of the present invention.

FIG. 3 is a flowchart showing steps of a method according to anotheraspect of the present invention.

FIG. 4 is a diagrammatic not-to-scale view of a second embodiment of alaser source according to another aspect of the present invention.

DETAILED DESCRIPTION

Various embodiments are now described with reference to the drawings,wherein like reference numerals are used to refer to like elementsthroughout. In the following description, for purposes of explanation,numerous specific details are set forth in order to promote a thoroughunderstanding of one or more embodiments. It may be evident in some orall instances, however, that any embodiment described below can bepracticed without adopting the specific design details described below.In other instances, well-known structures and devices are shown in blockdiagram form in order to facilitate description of one or moreembodiments.

With initial reference to FIG. 1 there is shown a schematic view of anexemplary EUV light source, e.g., an LPP EUV light source 20 accordingto one aspect of an embodiment of the present invention. As shown, theEUV light source 20 may include a pulsed or continuous laser source 22,which may for example be a pulsed gas discharge CO₂ laser sourceproducing radiation at 10.6 μm. The pulsed gas discharge CO₂ lasersource may have DC or RF excitation operating at high power and highpulse repetition rate. As noted, the laser source 22 may also includethe capability of delivering multiple pulses to a given volume of sourcematerial, including one or more pre-pulses.

The EUV light source 20 also includes a source delivery system 24 fordelivering source material in the form of liquid droplets or acontinuous liquid stream. The source material may be made up of tin or atin compound, although other materials could be used. The sourcedelivery system 24 introduces the source material into the interior of avessel or chamber 26 to an irradiation region 28 where the sourcematerial may be irradiated to produce plasma. In some cases, anelectrical charge is placed on the source material to permit the sourcematerial to be steered toward or away from the irradiation region 28. Itshould be noted that as used herein an irradiation region is a regionwhere source material irradiation may occur, and is an irradiationregion even at times when no irradiation is actually occurring.

Continuing with FIG. 1, the light source 20 may also include one or moreoptical elements such as a collector 30. The collector 30 may be anormal incidence reflector, for example, implemented as a multilayermirror (MLM), that is, a SiC substrate coated with a Mo/Si multilayerwith additional thin barrier layers deposited at each interface toeffectively block thermally-induced interlayer diffusion. Othersubstrate materials, such as Al or Si, can also be used. The collector30 may be in the form of a prolate ellipsoid, with an aperture to allowthe laser light to pass through and reach the irradiation region 28. Thecollector 30 may be, e.g., in the shape of a ellipsoid that has a firstfocus at the irradiation region 28 and a second focus at a so-calledintermediate point 40 (also called the intermediate focus 40) where theEUV light may be output from the EUV light source 20 and input to, e.g.,an integrated circuit lithography tool 50 which uses the light, forexample, to process a silicon wafer workpiece 52 in a known manner. Thesilicon wafer workpiece 52 is then additionally processed in a knownmanner to obtain an integrated circuit device.

An arrangement for the laser source 22 is shown in FIG. 2. As shown inFIG. 2, a main pulse from a laser (not shown) is caused to impinge on apulse combiner 100. The main pulse can come from any suitable laser suchas a CO₂ laser. Also in the arrangement of FIG. 2 a pre-pulse is appliedas described above. The source of the pre-pulse may be any suitablelaser such as a CO₂ or YAG laser (not shown). It will be readilyappreciated by one having ordinary skill in the art, however, that theteachings of this disclosure can be applied to systems in which nopre-pulse is used or is not always used.

The pulse combiner 100 is preferably made of zinc selenide (ZnSe) with amaximum metal reflector (MMR) coating if a pre-pulse from a CO₂ laser isused. If a pre-pulse from a YAG laser is used, then the pulse combiner100 is preferably made of a material having a very low coefficient ofthermal expansion such as fused silica or a titania-silicate glass suchas that sold under the trademarkULE® by Corning Glass Works, or alithium aluminosilicate glass-ceramic such as that sold under thetrademark ZERODUR® by Schott AG. Also if a pre-pulse from a YAG laser isused it is preferable to provide the pulse combiner 100 with a coatinghaving high reflectivity at 10.6 μm and high transmissivity at 1.064 μm.

The embodiment of FIG. 2 also includes a thermal lens compensating plate110. The thermal lens compensating plate 110 is preferably made of ZnSeand is preferably provided with a highly reflective coating. In thiscontext, highly reflective refers to a coating having less than about 2%absorption. One of ordinary skill in the art will appreciate that manymaterials can be used for the coating, such a copper, gold, ormolybdenum. The thermal lens compensating plate 110 may have a coatingon either its front or rear surface.

As described more fully below, the absorption of the thermal lenscompensating plate 110 is chosen to compensate for thermal lensing in adownstream lens 120 which may be a final focus lens for the system. Thelens 120 is preferably made of the same material as the thermal lenscompensating plate 110, e.g., preferably ZnSe. Also, cooling conditionsat the lens 120 and at the edge of the thermal lens compensating plate110 are preferably made substantially the same. As used herein,“temperature compensation” refers to establishing complementaryproperties so that the optical properties of the combined system of thethermal lens compensating plate 110 and the lens 120 are substantiallyindependent of temperature.

FIG. 2 shows an arrangement in which there are no intervening opticalelements between the thermal lens compensating plate and the lens 120.It will be readily appreciated by one having ordinary skill in the art,however that there could be intervening optical elements between thethermal lens compensating plate and the lens 120, for example, forpackaging or for geometrical layout purposes.

In the case of a pre-pulse from a CO₂ laser, most of the thermal lenswill occur in the lens 120 and the thermal lens compensating plate 110can provide compensation for both the lens 120 and the pre-pulsecombiner 100. In case of a pre-pulse from a YAG laser, the materialchoice for the pre-pulse combiner 100 should eliminate the thermal lensin the pre-pulse combiner 100 for the main pulse. The thermal lens inthe pre-pulse combiner 100 for the pre-pulse is less critical but can becompensated as well if design requirements compel such compensation.

As noted, the system made up of thermal lens compensating plate 110 andlens 120 preferably uses two optical elements made of the same material.One optical element, the lens 120, is used to focus the light onto theirradiation region 28 after it is steered by a steering mirror 130,preferably made of copper. The thermal lens created in this opticalelement is compensated by the thermal lens compensation plate 110. Asnoted, both of these optical elements may preferably be made from ZnSe,but other materials can be used, for example, ZnS, Ge, Other materials,for example, potassium chloride, sodium chloride, potassium bromide andso on.

The thermal lens compensation plate 110 acts as a mirror for the lightemitted by the CO₂ laser. The thermal lens of a lens such as the lens120 and the thermal lens of a mirror such as a thermal lens compensationplate 110 can be described mathematically. For the lens the relationshipis:

${\Delta({nl})}_{lens} = {{\left( \frac{\mathbb{d}n}{\mathbb{d}t} \right) \times L \times \Delta\; t} + {{CTE} \times n \times L \times \Delta\; t}}$where (dn/dt) is the change in refractive index with temperature for agiven lens material, L is the focal length of the lens, Δt is the changein temperature, CTE is the coefficient of thermal expansion for a givenlens material, and n is the refractive index for a given lens material.Using values for the example of a lens made using ZnSe this becomes:Δ(nl)_(lens)=6.1E-5×L×Δt+7.6E-5×2.4×L×Δt=7.9E-5×L×Δt

For the mirror the relationship isΔ(nl)_(mirror)=−2×CTE×L×ΔtUsing values for the example of a mirror made using ZnSe this becomes:Δ(nl)_(mirror)=−1.5E-5×L×Δt

In order to achieve substantially complete compensation, the ratio ofΔ(nl)_(lens)/Δ(nl)_(mirror)determines, other conditions being equal, arelative ratio for the absorption of the lens 120 and the reflectivecoating of the thermal lens compensation plate 110. Thus, for the samematerial (ZnSe in the example) and the same absorption, the thermal lensof the thermal lens compensation plate 110 acting as a mirror is7.9/1.5=5.3 times weaker than thermal lens of the lens 120. This meansthat for this example the absorption of the coating of the thermal lenscompensation plate 110 should be 5.3 times the absorption of the lens120 to achieve a compensating thermal lens effect, other conditionsbeing the same. With the absorption thus determined, the thermal lensfor the thermal lens compensation plate 110 should be the same inmagnitude as but opposite in sign from the thermal lens for the lens120, so that the former compensates for the later and reduces thetemperature dependency of the overall focusing system.

In some circumstances it may also be desirable to compensate for thechange in absorption of the lens 120 over its service lifetime. A lens120 made of ZnSe has a typical initial absorption of about 0.15 percent.This absorption typically doubles over the lifetime of such a finalfocus lens to about 0.3 percent. It is preferable to choose the coatingabsorption on the thermal lens compensation plate 110 to match theabsorption of the thermal lens 130 at the end of the lifetime of thethermal lens 130. This means that the coating on the thermal lenscompensation plate 110 will have absorption of at least 0.3*5.3=1.6%.Molybdenum with an optical absorption of about 2 percent would providean acceptably close match as a coating material.

If absorption of the coating on the thermal lens compensation plate 110is based on absorption of the lens 120 at the end of its lifetime thenit is preferable to adjust the thermal lens of the thermal lenscompensation plate 110 at the beginning and over the course of thelifetime of the system. One technique for adjusting the thermal lens ofthe thermal lens compensation plate 110 involves the use of a cold plate140 positioned behind the thermal lens compensation plate 110 as shownin FIG. 2. This cold plate 140 is adapted to provide substantiallyconstant uniform cooling over the whole surface of the thermal lenscompensation plate 110 with a controlled heat conductivity. The heatconductivity can be controlled by separating the cold plate 140 and thethermal lens compensation plate 110 by a gap 150 as shown and changingthe temperature of the cold plate 140 with a cold plate temperaturecontrol 160. This will reduce the thermal lens of the thermal lenscompensation plate 110 proportionally to the cooling flux. Thetemperature of the cold plate 140 is preferably set in the range ofabout −30° C. to about −10° C., and more preferably to about −10° C.when the lens 120 is first put into service and then gradually increasedas the lens 120 ages. Alternatively, the gap 150 can also be changed toreduce cooling.

FIG. 3 is a flowchart illustrating a method such as that just describedfor adjusting the amount of cooling applied to the thermal lenscompensation plate 110 over the operational lifetime of the focusingsystem. In a first step S1 the absorption of the thermal lenscompensation plate 110 is determined as set forth above using theexpected end-of-lifetime value for absorption for the lens 120. In astep S2 an initial temperature of the thermal lens compensation plate110 is determined which takes into account that the absorption of thethermal lens compensation plate 110 is high when compared to an initialvalue of the absorption of the lens 120 so that the thermal lenscompensation plate 110 would overcompensate if not cooled. In a step S3the cumulative time in use of the focusing system is determined. It willbe understood that this could be determined continuously or periodicallydepending on system design requirements. In a step S4 the amount ofcooling of the thermal lens compensation plate 110 is reduced as afunction of cumulative time as determined in step S3. In other words,the thermal lens compensation plate 110 is cooled less, that is, allowedto operate at progressively higher temperatures, as the system ages sothat its thermal lensing increases due to the rise in temperature as thethermal lensing of the lens 120 increases as its absorption increases.

The cold plate temperature control 160 may include a sensor for sensingthe temperature of the cold plate 140. Alternatively, the sensor in thecold plate temperature control 160 could be adapted to measure atemperature related to the temperature of the cold plate 140 such as thetemperature of the cooling water for the lens 120. Alternatively, thesystem can be calibrated from time to time and new temperature set pointcan be established through this calibration.

FIG. 4 shows a second embodiment of a thermal lens compensation systemaccording to the present invention. The embodiment of FIG. 4 can be usedwhen the main pulse and the pre-pulse propagate through the same beamdelivery system. Typically, the pre-pulse power is small compared to themain pulse power. Thus the thermal lens caused by the pre-pulse istypically small compared to the thermal lens caused by the main pulse.The thermal lens caused by the pre-pulse can, however, be compensatedfor as well if required by design considerations.

The embodiment of FIG. 4 is essentially the same as that of FIG. 2except that the main pulse and the pre-pulse propagate along a commonbeam path. Instead of a pre-pulse combiner 100 the embodiment of FIG. 3includes an optical wedge 170. If a pre-pulse from a CO₂ laser is usedthe optical wedge 170 is preferably made of ZnSe with a dichroiccoating, with a front surface that is highly reflective for the mainpulse and a back surface that is highly reflective to the pre-pulse. Ifa pre-pulse from a YAG laser is used, then the optical wedge 170 ispreferably made of fused silica or ZnSe. Also, it may be preferable tocurve the back surface of the optical wedge 170 to compensate forchromatic aberrations in the lens 120. Also, wedge 170 can have coolingon the back surface (not shown) to reduce its thermal lens.

The above description includes examples of one or more embodiments. Itis, of course, not possible to describe every conceivable combination ofcomponents or methodologies for purposes of describing theaforementioned embodiments, but one of ordinary skill in the art mayrecognize that many further combinations and permutations of variousembodiments are possible. Accordingly, the described embodiments areintended to embrace all such alterations, modifications and variationsthat fall within the spirit and scope of the appended claims.Furthermore, to the extent that the term “includes” is used in eitherthe detailed description or the claims, such term is intended to beinclusive in a manner similar to the term “comprising” as “comprising”is construed when employed as a transitional word in a claim.Furthermore, although elements of the described aspects and/orembodiments may be described or claimed in the singular, the plural iscontemplated unless limitation to the singular is explicitly stated.Additionally, all or a portion of any aspect and/or embodiment may beutilized with all or a portion of any other aspect and/or embodiment,unless stated otherwise.

What is claimed is:
 1. A method of temperature compensating a focusingsystem, the method comprising the steps of: regulating a temperature ofa thermal lens compensation plate to a first temperature, said firsttemperature being based on an optical absorption of said thermal lenscompensation plate, said optical absorption having been determined basedat least in part on an expected end-of-lifetime value for focus lensoptical absorption for a final focus lens comprising part of saidfocusing system; determining a value representative of cumulative timein use of said focusing system; and adjusting said temperature of saidthermal lens compensation plate to a second temperature higher than saidfirst temperature, said second temperature having a value based at leastin part on said cumulative time in use.
 2. A method as claimed in claim1 wherein said determining step and said adjusting step are carried outperiodically.
 3. A method as claimed in claim 1 wherein said determiningstep and said adjusting step are carried out substantially continuously.